A photoconductor's conductance varies linearly with the intensity of incident radiation upon the photoconductor. This change in conductance is used to create a change in current which is amplified to measure this incident radiation.
A photoconductor requires a certain bias voltage and current to operate in its linear region. Ideally, a constant voltage is fixed across the photoconductor so that any change in conductance would create a corresponding linear change in current through the photoconductor.
FIG. 1 illustrates the conventional method of biasing a photoconductor 103 and amplifying its signal. In this circuit a voltage 100 is supplied across two resistors 101, 102 connected in series with the photoconductor 103. Any change in the conductance of the photoconductor 103 would change the voltage across the photoconductor 103. Since the sum of resistors 101, 102 is large relative to the resistance of the photoconductor 103, a change in photoconductor conductance would not significantly change the bias current through it.
A capacitor 104 at the node between the series resistors 101, 102 decouples power supply noise and must be fairly large since voltage fluctuations from the power supply couple into the amplifier 110.
A capacitor 105 is used to couple an a.c. signal from the photoconductor 103 to a low impedence amplifier 110. This capacitor 105 must also be large to filter direct current and yet allow low frequency signals to pass into the amplifier 110 relatively unattenuated.
The drawbacks of this conventional biasing and amplifying circuit are: (1) the capacitors 104, 105 in the circuit must have a high capacitance and are therefore expensive and bulky; (2) the bias voltage changes with low frequency signals since, at low frequencies, the low impedence amplifier is no longer in parallel with the photoconductor due to the attenuation of the coupling capacitor 105; (3) when the photoconductor is removed, the decoupling and coupling capacitors 104, 105 charge up to the supply voltage 100 and could inject damaging surge currents into a photoconductor when it is inserted in the circuit; and (4) when the photoconductor responds to a large signal, the voltage across it drops, producing a nonlinear incremental response to additional radiation.